Pluse wave velocity measurement system

ABSTRACT

A system and method is disclosed for measurement of pulse wave velocity of a vessel. An intravascular device comprises a first and a second marker provided at different locations along the length of the intravascular device of which positions are localizable by a tracking apparatus. The intravascular device provides plurality of measurements along the length of the vessel, while the intravascular device is moved from a first position to a second position, corresponding to a first and a second time. At the second time the position of the first marker in the vessel corresponds to the position of the second marker at the first time. The pulse wave velocity value of the vessel is ascertained based on measurements associated for the first time and the second time from the plurality of measurements and based on the distance between the locations of the two markers along the length of the intravascular device.

FIELD OF THE INVENTION

The present invention relates to a medical system and method ofmeasuring pulse wave velocity of a vessel with the medical system.

BACKGROUND OF THE INVENTION

Pulse wave velocity (PWV) measurements are a well-established way tomeasure the compliance of major blood vessels. Aortic and cardiac PWVare used to evaluate the risk of cardiovascular events while the renalPWV may help in patent stratification for renal denervation.

There are two mainstream ways of measuring PWV:

a) By measuring flow and pressure at (approximately) the same locationand deriving PWV from the relationship between the change in pressureand the change in flow velocity (e.g. Khir et. al., Determination ofwave speed and wave separation in the arteries, Journal of Biomechanics34, 2001, 1145-1155; and Davies et. al., Use of simultaneous pressureand velocity measurements to estimate arterial wave speed at a singlesite in humans, Am J Physiol Heart Circ Physiol 290: H878-H885, 2006);

b) by timing the rise of a wave or pressure pulse over a known distance(Millasseau et. al., Evaluation of Carotid-Femoral Pulse Wave Velocity:Influence of Timing Algorithm and Heart Rate, Hypertension. 2005;45:222-226; and Harbaoui et. al., Development of Coronary Pulse WaveVelocity: New Pathophysiological Insight Into Coronary Artery Disease,Journal of the American Heart Association 2017; 6:e004981). Both methodshave their advantages and disadvantages. Measuring PWV via thepressure-flow-relationship typically requires an intravascularflow-pressure-wire. Timing the pressure or flow wave is technically lessdemanding but works best for long blood vessels like the aorta, wheredelay times are long and comparatively easy to measure. However, forapplications like patient stratification for renal denervation, it isnecessary to measure PWV in blood vessels only a few cm long, such asthe renal artery.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a medical system formeasuring PWV in blood vessels with improved accuracy andreproducibility.

The system comprises an intravascular guidewire/catheter with a singlepressure or flow sensor and a processing unit configured to derive adelay time between an electrocardiogram (ECG) signal or additionalpressure signal and the pressure/flow signal measured by the sensorwhereby the intravascular device comprises special markings near thesensor that allow an imaging or tracking modality (angiography,ultrasound, electro-magnetic tracking) to determine a pullback distancewith a high degree of accuracy.

In an embodiment, the system for measuring PWV in blood vesselscomprises an intravascular guidewire/catheter with a single pressure orflow sensor and a processing unit to derive a delay time between an ECGor additional pressure signal and the pressure/flow signal measured bythe sensor whereby the intravascular device comprises at least twomarkings at and proximally from the sensor, wherein the at least twomarkings form a ruler that allows an imaging or tracking modality(angiography, ultrasound, electro-magnetic tracking) to determine apullback distance with a high degree of accuracy independently of thealignment between imaging plane and sensor.

In an aspect of the invention a method of measurement of pulse wavevelocity of a vessel is presented, wherein the method comprises:

receiving plurality of measurements along the length of the vessel froman intravascular device, the intravascular device comprising two markersprovided at different locations along the length of the intravasculardevice of which positions are localizable by a tracking apparatus;

receiving position information of the two markers from the trackingapparatus;

associating for a first time a measurement of the plurality ofmeasurements with the positions of the markers, wherein the first markeris in a first position and the second marker is in a second position,

detecting upon movement of the intravascular device from the positioninformation when the first marker is reaching the second position andassociating a second time;

ascertaining the pulse wave velocity value of the vessel based onmeasurements associated for the first time and the second time from theplurality of measurements and based on the distance between thelocations of the two markers along the length of the intravasculardevice.

Additional aspects and advantages of the invention will become moreapparent from the following detailed description, which may be bestunderstood with reference to and in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows exemplarily typical pressure signals (pressure versus timeover one heartbeat).

FIG. 2 illustrates schematically and exemplarily the general concept ofthe system according to the invention.

FIG. 3 shows schematically and exemplarily a measurement procedureaccording to the invention.

FIG. 4 shows schematically and exemplarily an alternative measurementprocedure according to the invention.

FIG. 5 shows schematically the functional use of the system according tothe invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Pulse wave velocity can be determined by measuring the time delay Δt ofa pressure or flow velocity wave when travelling a distance Δx as:

$\begin{matrix}{{PWV} = \frac{\Delta\; t}{\Delta\; x}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

It is therefore possible to measure PWV with two (pressure or flow)sensors located at a known distance from each other. FIG. 1 shows howsuch a double measurement can be used to determine Δt. However, thisrequires an intravascular device with two pressure sensors. Such adevice is currently not commercially available for human use. It wouldalso probably cost twice as much as a standard pressure or flow wirewith a single sensor and the need to comprise connective cables for twosensors would likely compromise the mechanical properties of the device.

It is also possible to measure the time delay Δt of a pressure or flowvelocity wave with a single sensor (pressure or flow), by pulling backthe sensor along the vessel to measure at different locationssequentially. In this case one needs a reference signal from which totime each pulse. Typically, one uses the ECG signal or the signal from acentral pressure sensor. The method is described in detail in Harbaouiet. al. This method is usually advantageous because it only requires asingle pressure or flow sensor.

However, using a single sensor can significantly decrease accuracy,especially for measurements in short arteries. This is because the needto move the sensor adds inaccuracy in the distance Δx betweenmeasurement locations, while for a two-sensor probe the distance betweensensors is mechanically fixed with very little error. For measurementsin long arteries, the relative error in distance can typically beneglected, but in arteries that are only a few centimeters long, theerror quickly becomes unacceptably large. For example, if one measuresPWV in the aorta over a distance of Δx=40 cm then an error of ±1 mm inthe placement of each sensor only causes a relative error of 0.5%. Butif one measures in a short artery like the renal artery where thedistance between measurement points is only Δx=2 mm, then the sameabsolute error of ±1 mm in the placement of each sensor already causes arelative error of 10%.

This problem is made worse because it is difficult to measure thelocation of the sensor in the vessel with an accuracy of ±1 mm orbetter. Harbaoui et. al. try to overcome the issue by measuring thedisplacement of the external part of the device during pullback. This isunfortunately not accurate enough for measuring PWV in short arteries.The reason for this is that any intravascular device (guide wire,catheter, etc.) when advanced deeper into a vessel will follow thevessel wall and bunch up. Subsequently, during pullback the shaft of thedevice will straighten. Also during pullback, parts of the device cancatch or snag at the vessel walls leading to a non-smooth movement ofthe device tip. As a consequence, the displacement of the sensorintegrated at the tip portion of the device will typically be less thanthe displacement of the external shaft during pullback. This uncertaintyscales with the total length of the device inside the patient and ismost problematic for measurement of PWV in short arteries. For example,if a device has 1 m of its length inside the patient, and if there is0.5% deviation between the displacement of the distal tip and thedisplacement of the external shaft, then there will be a relative errorof 25% when measuring Δx inside a 2 cm long renal artery, but the errorwill only be 2.5% when measuring Δx inside a 20 cm long artery.

Alternatively, one could try to determine the sensor displacement byusing imaging. Angiography is typically used to navigate a sensor wireor catheter because the physician can observe the movement of the deviceand sensor inside the blood vessel directly. Unfortunately, the movementseen on a two-dimensional (2D) angiogram is only the projection of theactual three-dimensional (3D) movement onto a 2D plane. If the C-arm isnot perfectly aligned with the blood vessel and the device, or if theblood vessel is not perfectly straight, this can introduce significantinaccuracies.

For the reasons given above, PWV measurement with a single sensor (andmanual pullback) is currently not reliable in short arteries. Toincrease the accuracy of these measurements, it is necessary to increasethe accuracy of the sensor displacement measurement. This inventiondescribes a way to solve this issue.

For the general concept, a schematic and exemplary system 100 isillustrated in FIG. 2, wherein for various advantageous embodiments ofthe system the functionality of an element may be fulfilled by variousdifferent devices or sub-systems, as it will be elucidated in thedescription. The system 100, which may be referred to as pulse wavevelocity measurement system, may be used for patient stratification fortreatment purposes. For example, the PWV value in the renal arteries maybe utilized to determine whether a patient is suitable for renal arterydenervation. Based on the PWV determination, the intravascular system100 may be used to classify one or more patients into groupsrespectively associated with varying degrees of predicted therapeuticbenefit of renal denervation. Any suitable number of groups orcategories are contemplated. For example, the groups may include groupsrespectively for those patients with low, moderate, and/or highlikelihood of therapeutic benefit from renal denervation, based on thePWV. Based on the stratification or classification, the system 100 canrecommend the degree to which one or more patients are eligiblecandidates for renal denervation.

The vessel 80 may represent fluid-filled structures, both natural andartificial ones. The vessel 80 may be within a body of a patient. Wallsof the vessel 80 define a lumen 81 through which fluid flows within thevessel 80. The vessel 80 may be a blood vessel, as an artery or a veinof a patient's vascular system, including cardiac vasculature,peripheral vasculature, cerebral vasculature, renal vasculature, and/oror any other suitable lumen inside the body. For example, theintravascular device 10 may be used to examine any number of anatomicallocations in vessels of organs including the liver, heart, kidneys, gallbladder, pancreas, lungs, intestines, brain, urinary tract. In additionto natural structures, the intravascular device 10 may be used toexamine artificial structures such as, but without limitation, grafts,stents, shunts.

The vessel 80 may be located within a body portion. When the vessel 80is the renal artery, the patient body portion may include the abdomen,lumbar region, and/or thoracic region. In some examples, the vessel 80may be located within any portion of the patient body, including thehead, neck, chest, abdomen, arms, groin, legs, etc.

The intravascular device 10 may include a flexible elongate member 11such as a catheter, guide wire, or guide catheter, or other long, thin,long, flexible structure that may be inserted into a vessel 80 of apatient. The vessel 80 may be a renal artery as shown in FIGS. 3 and 4.The intravascular device 10 of the present disclosure has a cylindricalprofile with a circular cross-section that defines an outer diameter ofthe intravascular device 10, in other instances, all or a portion of theintravascular device may have other geometrical cross-sectional profiles(e.g., oval, rectangular, square, elliptical, etc.). The intravasculardevice 10 may or may not include a lumen extending along all or aportion of its length for receiving and/or guiding other instruments. Ifthe intravascular device 10 includes a lumen, the lumen may be centeredor offset with respect to the cross-sectional profile of theintravascular device 10.

The intravascular device 10, or the various components thereof, may bemanufactured from a variety of materials, including, by way ofnon-limiting example, plastics, polytetrafluoroethylene (PTFE),polyether block amide (PEBAX), thermoplastic, polyimide, silicone,elastomer, metals, such as stainless steel, titanium, shape-memoryalloys such as Nitinol, and/or other biologically compatible materials.In addition, the intravascular device may be manufactured in a varietyof lengths, diameters, dimensions, and shapes, including a catheter,guide wire, a combination of catheter and guide wire, etc. For example,the flexible elongate member 11 may be manufactured to have lengthranging from approximately 115 cm-195 cm. The flexible elongate member11 may be manufactured to have length of approximately 135 cm. Theflexible elongate member 11 may be manufactured to have an outertransverse dimension or diameter ranging from about 0.35 mm-2.67 mm (1Fr-8 Fr). The flexible elongate member 11 may be manufactured to have atransverse dimension of 2 mm (6 Fr) or less, thereby permitting theintravascular device 10 to be configured for insertion into the renalvasculature of a patient. These examples are provided for illustrativepurposes only, and are not intended to be limiting. In some examples,the intravascular device is sized and shaped such that it can be movedinside the vasculature (or other internal lumen(s)) of a patient suchthat at least one of pressure, volumetric flow, flow velocity, vesseldiameter and wall thickness of a vessel can be monitored from within andalong a length of the vessel.

The intravascular device 10 includes a sensor 16 at the distal portionof the flexible elongate member 11. Alternatively or additionally,further sensors may be included along the length of the flexibleelongate member. The intravascular device comprises two markers 12 and14, provided at different locations along the flexible elongate member,and of which positions within the vessel 80 are localizable by atracking apparatus 40. The system further comprises an external console,or apparatus 30, to receive the plurality of measurements provided bythe intravascular device 10, to process the measurement signals by aprocessor 31, and optionally or alternatively to store the receivedmeasurement signals and/or processed measurement signals in a memoryunit 32. The measurements provided by the intravascular device 10 maydirectly be transmitted to the external console 30, or alternativelythey may be transmitted through a patient interface module 20 to whichthe intravascular device is connected. The patient interface module maytransmit the measurement information to the external console either bywired or wireless connection. Optionally, the system comprises anelectrocardiogram (ECG) unit 50 for measurement of internal electrogramor external electrocardiogram signals, which are transmitted to theapparatus 30 for using ECG signals, when required, for ascertainingpulse wave velocity value, in combination with the plurality ofmeasurement signals received from the intravascular device 10. Theinternal electrogram signals may be measured by an electrode placed onan intravascular device that is introduced in a vessel, while theexternal electrocardiograms may be measured by external leads attachedto the body of the patient.

In general, sensor 16 may provide one type of measurements or multipletypes of measurements from the group of pressure measurements, flowmeasurements, vessel diameter measurements and vessel cross-sectionmeasurements. The multiple types of measurements with the same sensormay be performed simultaneously or interspersed. An example of suchsensor is a capacitive micromachined sensor, which can emit and receiveultrasound waves, while from its design comprises a cavity closed by amembrane that upon pressure can vary capacitance value and hence canprovide pressure measurements. Alternatively, the pressure value can bedetermined from the frequency characteristics at which the ultrasoundemission and/or reception occurs, which are pressure sensitive. Once thecapacitive micromachined sensor is operated to emit and receiveultrasound waves, ultrasound imaging of the vessel can be performed, andflow velocity can be measured by using ultrasound-Doppler principle.Vessel diameter and/or vessel cross-section can be determined from theultrasound image, and having additionally the flow velocity of blood inthe vessel, the volumetric flow can be calculated.

In the specific embodiment, schematically illustrated in FIG. 3, thesystem comprises an intravascular guidewire with a single pressuresensor at or near the distal tip. In the proximity of the pressuresensor a first radiopaque marker is placed. A second radiopaque markeris placed proximally to the first marker, at a fixed distance Δx, withΔx in the range of 0.5 cm to 4 cm for an intravascular device configuredfor measurements in the renal arteries. The system further comprises theapparatus 30 to read out the pressure pulses and to derive a pulse delaytime ΔT using ECG signal or alternatively, stationary pressure or flowsensor signal as a reference. The system further comprises a radiology(x-ray) system or apparatus suitable for angiographic imaging withfunctionality of the above mentioned tracking apparatus 40, and whichadditionally may provide morphological information (image) of the vesselanatomy. Alternatively, computer tomography (CT) or magnetic resonanceimaging (MRI) may be used for acquiring three-dimensional morphologicalinformation (anatomy of the vessel) while tracking the markers. For MRItracking coils integrated in the intravascular device may be used.

To measure pulse wave velocity, the physician places the intravasculardevice into the blood vessel under investigation so that the angiogramshows both radiopaque markers inside the blood vessel. Because theguidewire between the two markers is sufficiently stiff, the distancebetween both markers is known with a high degree of precision: which isΔx. The position of the second marker on the radiologic or angiographicimage is recorded or labeled with a marking, either automatically by thesystem or manually by the physician. The physician will then measure thepulse delay time ΔT₁ at this position, typically by averaging overmultiple heartbeats. After this first measurement, the physician thengently pulls back the intravascular device until the new position of thefirst radiopaque marker on the image coincides with the marking showingthe old position of the second radiopaque marker. This indicates adisplacement of the sensor 16 by distance Δx from the first measurementlocation. The physician will now measure the pulse delay time ΔT₂ atthis new position, averaging over the same number of heartbeats asbefore.

The processor will then calculate and optionally display PWV as:

$\begin{matrix}{{PWV} = \frac{{\Delta\; T_{1}} - {\Delta\; T_{2}}}{\Delta\; x}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

The second embodiment of the system, illustrated in FIG. 4, is similarto the first embodiment, except that the guidewire/catheter tipcomprises multiple radiopaque markers proximally from the sensor atfixed, known distances from each other. Because the guidewire betweenthe markers is sufficiently stiff, these markers form a kind of rulerthat is visible under radiologic and/or angiographic imaging.

To measure pulse wave velocity, the physician places the intravasculardevice into the blood vessel under investigation, so that the pressuresensor is located as distally as practicable. The physician will thenmeasure the pulse delay time ΔT₁ at this position, typically byaveraging over multiple heartbeats. During the measurement, the positionof the radiopaque markers on the radiologic or angiographic image isrecorded automatically and the distances between markers on the imageare compared with the true distances of the markers. The ratio ofapparent distances and true (or physical) distances provides a localcorrection factor for the processing unit that corrects for the localangle between the intravascular device and its projection on theradiologic or angiographic image, which is caused due to bending of thevessel or the shaft. After the first measurement, the physician thengently pulls back the intravascular device towards the proximal portionof the vessel under investigation. The sensor still has to be within thepart of the vessel previously covered by the radiopaque markers. Thephysician will now measure the pulse delay time ΔT₂ at this newposition, averaging over the same number of heartbeats as before. Duringthe measurement, the position of the radiopaque markers on the angiogramis again recorded automatically and the new locations of the radiopaquemarkers are compared with their old positions. Using the correctionfactors derived for the first measurement, the processing unit uses thisinformation to calculate a corrected distance Δx between the positionsof the sensor at the first and second measurement instances.

The system will then calculate and display PWV according to Eq. 2.

The third embodiment of the system is either one of the first and secondembodiments of the system, wherein the radiology apparatus is replacedwith an ultrasound imaging system and the radiopaque markers arereplaced by markers that are visible under ultrasound imaging.

In a forth embodiment of the system, which is a further alternative ofthe third embodiment, the markers are active or passive ultrasoundtransducers that emit or receive, respectively, ultrasound waves to orfrom an ultrasound probe, which can be a further ultrasound deviceconfigured to be introduced into the body of the subject or anextracorporeal ultrasound probe, wherein the locations of the active orpassive ultrasound markers are tracked on the ultrasound image providedby the further ultrasound device or the extracorporeal ultrasound probe.

In a fifth embodiment of the system, which is a further alternative ofthe fourth embodiment of the system, the sensor is an ultrasound flowsensor and is simultaneously used as active ultrasound location marker.

In a sixth embodiment of the system, which may be based on the first orsecond embodiments of the system, the radiology apparatus is replacedwith an electro-magnetic tracking system and the radiopaque markers arereplaced by electromagnetic coils that are traceable with theelectro-magnetic tracking system.

In a seventh embodiment of the system, which may be based on any of thefirst to the sixth embodiments of the system, the location of themarkers is continuously tracked during the measurements to correct formovement during the measurement such as breathing movement or patientmovement.

In an eighth embodiment of the system, which may be based on any of thefirst to the seventh embodiments of the system, the system uses a vesseldiameter measurement sensor like an intravascular ultrasound (IVUS)sensor instead of a pressure or flow sensor.

In a ninth embodiment, which is based on the eighth embodiment of thesystem, the IVUS sensor is used simultaneously as active ultrasoundlocation marker.

In any of the relevant embodiments the system can be configured toascertain the pulse wave velocity value of the vessel based on theplurality of measurements along the length of the vessel acquired asdescribed above, wherein the plurality of measurements may alternativelybe any of the combinations: pressure and vessel diameter measurements,pressure and flow velocity measurements, pressure and volumetric flowmeasurements, flow velocity and vessel diameter measurements, pressureand vessel wall thickness measurements. Reference is made to WO2017/198800 A1, WO 2017/198867 A1, WO 2017/198490 A1, WO 2017/198871 A1,for description of using specific measurement combinations forcalculation of pulse wave velocity.

For any of the embodiments where a morphology of the vessel is availablebesides the position of the markers within the vessel, the accuracy ofthe PWV calculation can further be improved by making the apparentdistance Δx in Eq. 1 or Eq. 2 for PWV measurements, be the distance fromthe first marker to the second marker projected on the axis of thevessel. For small distances, the apparent distance between the markers(which is needed for PWV measurements) can be different from thephysical distance if the markers are not in the same radial positionwith respect to the axis of the vessel. This is an additional benefitcompared to dual sensor measurements.

FIG. 5 illustrates schematically the functional use of any of theembodiments of the above systems according to the invention. The method200 of measurement of pulse wave velocity of a vessel starts with step202 of receiving plurality of measurements along the length of thevessel from an intravascular device. As already described for thesystems, the intravascular device comprises at least two markersprovided at different locations along the length of the intravasculardevice of which positions are localizable by the tracking apparatus. Instep 204 the system receives position information of the two markersfrom the tracking apparatus and the system associates in step 206 for afirst time a measurement of the plurality of measurements with thepositions of the markers, wherein the first marker is in a firstposition and the second marker is in a second position. In step 208 thesystem detects, upon movement of the intravascular device within thevessel, from the position information when the first marker is reachingthe second position and then associating a second time. In step 210 thepulse wave velocity value of the vessel is ascertained based onmeasurements associated for the first time and the second time from theplurality of measurements and based on the distance between thelocations of the two markers along the length of the intravasculardevice.

In step 212 the pulse wave velocity value is output to a display.Alternatively or additionally, the pulse wave velocity value may becompared automatically by the processor to a threshold value fordetermining whether the patient is eligible for treatment or not, andthe indication of eligibility is output to the display. Clinical studiesprovided evidence that any value of the pulse wave velocity for renalarteries above a threshold of about 9 m/s would make the patienteligible for renal denervation treatment with sufficient benefit inblood pressure reduction.

By repeating the steps 202 to 212 for successive portions of the vessel,a PWV map along the vessel can be generated and displayed as overlay on,or adjacent to, the morphological information of the vessel, which maybe a two- or three-dimensional image of the vessel acquired byangiography or other imaging modality, such as for example ultrasoundimaging. The eligibility of the patient for renal denervation treatmentwith sufficient benefit in blood pressure reduction can then be assessedbased on a more comprehensive set of information provided by the PWV mapalong the vessel.

Although the invention is primarily described exemplarily formeasurement of pulse wave velocity in renal arteries, it is applicablefor measurement of pulse wave velocity in any other vessel, includingcardiovascular and cerebral vessels.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims.

A single unit or device may fulfill the functions of several itemsrecited in the claims. The mere fact that certain measures are recitedin mutually different dependent claims does not indicate that acombination of these measures cannot be used to advantage.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality.

Any reference signs in the claims should not be construed as limitingthe scope.

1. A system for measurement of pulse wave velocity of a vessel,comprising: an intravascular device configured to provide plurality ofmeasurements along the length of the vessel, wherein the intravasculardevice comprises two markers provided at different locations along thelength of the intravascular device of which positions are localizable bya tracking apparatus; an apparatus configured to: receive the pluralityof measurements from the intravascular device; receive positioninformation of the two markers from the tracking apparatus; associatefor a first time a measurement of the plurality of measurements with thepositions of the markers, wherein the first marker is in a firstposition and the second marker is in a second position; detect uponmovement of the intravascular device from the position information whenthe first marker reaches the second position and associate a secondtime; ascertain the pulse wave velocity value of the vessel based onmeasurements associated for the first time and the second time from theplurality of measurements and based on the distance between thelocations of the two markers along the length of the intravasculardevice.
 2. The system according to claim 1, wherein the intravasculardevice comprises a sensor at its distal portion, in proximity of one ofthe two markers, configured to provide the plurality of measurementsalong the length of the vessel.
 3. The system of claim 1, wherein theapparatus is further configured to: ascertain an apparent distancebetween the first marker and the second marker at the first time fromthe position information received from the tracking apparatus; comparethe apparent distance with the physical distance between the locationsof the markers on the intravascular device; ascertain a local correctionfactor based on the apparent distance and the physical distance;ascertain a corrected pulse wave velocity value based on the pulse wavevelocity value and the correction factor.
 4. The system of claim 1,wherein the intravascular device comprises a plurality of markerslocalizable by the tracking apparatus and provided at differentlocations along the length of the intravascular device, wherein theplurality of markers form a ruler.
 5. The system of claim 1, furthercomprising a user interface configured to display the pulse wavevelocity value.
 6. The system of claim 1, further comprising thetracking apparatus configured to localize the positions of the markers.7. The system of claim 6, wherein the tracking apparatus is a radiologyapparatus and the markers comprise radiopaque material.
 8. The system ofclaim 1, wherein the plurality of measurements along the length of thevessel comprise at least one of the types of pressure measurements, flowmeasurements, vessel diameter measurements and vessel cross-sectionmeasurements.
 9. The system of claim 6, wherein the tracking apparatusis an ultrasound probe and wherein the markers are active or passiveultrasound transducers that emit or receive, respectively, ultrasoundwaves to or from the ultrasound probe.
 10. The system of claim 9,wherein the ultrasound probe is an extracorporeal ultrasound probe. 11.The system of claim 9, wherein at least one of the marker is used as thesensor to provide the plurality of measurements along the length of thevessel.
 12. The system of claim 11, wherein the sensor is configured toprovide the plurality of measurements comprising flow measurements. 13.The system of claim 11, wherein the sensor is configured to provide theplurality of measurements comprising vessel diameter or cross-sectionmeasurements.
 14. The system of claim 6, wherein the tracking apparatusis electromagnetic tracking apparatus and wherein the markers comprisepermanent magnetic material or electromagnetic coils.
 15. A method ofmeasurement of pulse wave velocity of a vessel, comprising: receivingplurality of measurements along the length of the vessel from anintravascular device, the intravascular device comprising two markersprovided at different locations along the length of the intravasculardevice of which positions are localizable by a tracking apparatus;receiving position information of the two markers from the trackingapparatus; associating for a first time a measurement of the pluralityof measurements with the positions of the markers, wherein the firstmarker is in a first position and the second marker is in a secondposition; detecting upon movement of the intravascular device from theposition information when the first marker reaches the second positionand associating a second time; ascertaining the pulse wave velocityvalue of the vessel based on measurements associated for the first timeand the second time from the plurality of measurements and based on thedistance between the locations of the two markers along the length ofthe intravascular device.